The present invention relates in general to switched bandwidth digital filters using swapped coefficient sets, and more specifically, to slewing a subset of the coefficients to reduce audible effects during switching.
Digital filters manipulate discrete samples of an input signal to produce a filtered output signal. Various filter structures are known in the art, such as those for finite impulse-response (FIR) filters and infinite impulse-response (IIR) filters. Higher order IIR filters (providing more selectivity) are typically implemented using a plurality of lower order filters connected in cascade.
During processing of a signal, it may become necessary to change the filtering of the signal (e.g., a change in bandwidth, passband characteristic, group delay, or other filter parameters). To minimize hardware and/or software requirements, it is desirable to use the same filter structure before and after the change by merely changing the digital filter coefficients.
In a digital signal processing (DSP) radio receiver, for example, a digital channel filter is applied to an intermediate frequency (IF) signal to select the desired signal and reject other broadcast signals. A wide or a narrow passband may be used for the channel filter depending upon the presence of interfering adjacent or alternate channel broadcast signals. When switching between the two bandwidths by switching between two coefficient sets in a DSP filter, the sudden change in bandwidth may be noticeable to the listener and may lead to a perception of poor quality.
It would be possible to gradually slew the bandwidth between wide and narrow, but the computation and/or storage requirements for updating all the coefficients of the filter during slewing would be too great and would be expensive.
The present invention has the advantage of eliminating a sudden bandwidth change without slewing a large number of coefficients within the filter.
In one aspect of the invention, a method is provided for slewing between a first bandwidth characteristic and a second bandwidth characteristic in a switched bandwidth digital filter including a plurality of second-order filter sections. Each of the sections comprises respective coefficients having a first set of values in providing the first bandwidth characteristic and having a second set of values in providing the second bandwidth characteristic. The coefficients of a selected filter section are slewed between the first set of values and a surrogate set of values while coefficients in the filter sections other than the selected filter section are held constant. The coefficients of the filter sections other than the selected filter section are swapped from their respective first set of values to their respective second set of values. When going from wideband to narrowband, the slewing step may preferably be performed before the swapping step. For going from narrowband to wideband, the swapping step may preferably be performed before the slewing step. In each case, the slewing/swapping order might sometimes be reversed, depending on analysis and/or subjective listening, and depending on how easy it is to find a good surrogate set of values.
The selected filter section is preferably the one section which has the most critically damped characteristic (i.e., most flat within the passband). The surrogate coefficients of the selected section are designed, in combination with the remaining unchanged sections, to provide a passband characteristic which approximates an overall passband of said digital filter when it is providing its narrower bandwidth characteristic.